According to the United States Census Bureau, the township has a total area of 23.5 square miles (60.9 km2), of which 23.2 square miles (60.1 km2) is land and 0.3 square miles (0.9 km2), or 1.43%, is water.[4]

When Cumberland County was founded in 1750, it originally consisted of three townships: East and West Pennsborough Townships, and Middleton Township. In 1810, due to population increase, Middleton Township was divided into North Middleton Township to the north of Carlisle, and South Middleton Township to the south. In the mid 18th century, the area that is now the township was inhabited primarily by Scotch Irish settlers who were granted land patents in that region by the family of William Penn, the proprietors of Pennsylvania at the time. The rapid settlement of this area at that time was the result of the Penns' plan to keep the Scotch Irish who were viewed as rowdy and unsophisticated out of the more refined Lancaster and York counties to the southeast.[citation needed] Thus, by the late 1700s, the population west of the Susquehanna River was composed almost exclusively of Scotch Irish settlers.

As was common of most areas in the Cumberland Valley at that time, most early settlers of North Middleton Township were farmers by trade, and perhaps the area's most lasting legacy was the great quantity and quality of barns built there. The majority of these barns exhibited a style that came to be known as the Pennsylvania barn. Many of these barns were built during the late 1800s, but a fair number still exist in the township today. According to a township publication from 1976, there were 78 barns still standing in the area,[5] and although this number has dwindled since that time, there are still several dozen barns in the township today, and agriculture still remains a substantial sector in the region's economy.

There were 4,039 households, out of which 31.4% had children under the age of 18 living with them, 61.6% were married couples living together, 8.2% had a female householder with no husband present, and 27.0% were non-families. 22.2% of all households were made up of individuals, and 8.4% had someone living alone who was 65 years of age or older. The average household size was 2.48 and the average family size was 2.89.

In the township the population was spread out, with 23.3% under the age of 18, 6.8% from 18 to 24, 29.0% from 25 to 44, 27.0% from 45 to 64, and 13.9% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 94.0 males. For every 100 females age 18 and over, there were 91.1 males.

The median income for a household in the township was $50,010, and the median income for a family was $56,846. Males had a median income of $37,880 versus $25,791 for females. The per capita income for the township was $22,947. About 1.4% of families and 2.5% of the population were below the poverty line, including 1.9% of those under age 18 and 4.0% of those age 65 or over.